This invention relates to a magnetron constructed to prevent generation of noise.
A magnetron generating a microwave at a high efficiency is widely used in various high frequency apparatus, such as a microwave range, a radar and the like.
FIG. 1 shows a longitudinal sectional view of a prior art magnetron comprising an anode cylinder 11 provided with a plurality of radial vanes 12 secured to the inner wall thereof. At the center of the anode cylinder 11 is disposed a cathode structure 13. Frustum shaped pole pieces 14 and 15 are disposed on the opposite ends of the anode cylinder. An antenna 17 extends through the pole piece 14 between the space in which the vanes 12 are located, and an output terminal 16 and three load conductors 19, 20 and 21 connected to the cathode structure 13 extend upwardly through the center of the other pole piece 15. Permanent magnets 23 and 24 are disposed close to the pole pieces 14 and 15 respectively and flux focusing rings 26 and 27 are disposed between the permanent magnet 23 and the pole piece 14 and between the permanent magnet 24 and the pole piece 15. A plurality of heat radiating fins 28 are provided to surround the anode cylinder 11 and the component parts described above are contained in an iron casing 29. A casing 31 containing a filter 32 for suppressing the unwanted electromagnetic wave leakage propagating through the input terminal of the cathode voltage is secured to the top of the casing 29. A gasket 33 is mounted on the inner periphery of the casing 29 and a stem 35 is provided at the top center of the casing 29 for supporting the lead conductors 19, 20 and 21. A support 36 for supporting the stem 35 is also mounted on the magnetic pole piece 15. The vanes 12 and the cathode structure 13 define an interaction space 37 and on both ends thereof are defined end spaces 38 and 39 by opposite ends of the anode cylinder 11, vanes 12 and pole pieces 14 and 15.
When an input is supplied to the cathode structure 13 through the filter 31, the electrons emitted by the cathode structure undergo a whirling motion whereby a microwave energy is radiated through antenna 17 and output terminal 16 which is utilized to cook foodstuffs in a microwave range.
Although most of the microwave energy generated in this manner comprises the fundamental wave, many unwanted waves such as second, third and other higher harmonics as well as sideband frequencies are also generated. Furthermore, in the prior art construction the end spaces 38 and 39 are merely designed to cause the magnetic flux from the pole pieces 14 and 15 to flow in parallel through the interaction space 37 without regard to the resonance frequency in the end spaces 38 and 39 so that the resonance frequency usually has a resonance point between 3.5 to 4.5 GHz. For this reason, all unwanted waves, especially higher harmonic waves having a large energy leak to the outside via antenna 17 or the cathode structure 13 thus affecting nearby communication apparatus or television receiving sets. Although filter 31 is provided for the cathode structure 13 for preventing such leakage of the unwanted electric waves, it is impossible to perfectly prevent the leakage. Among the higher harmonics the second harmonic has the largest energy thus creating the most serious problem.